Plasma generating devices play an important role in many areas. For example, plasma is used in displays, such as television sets and computer monitors, spectrography, in spraying applications such as coating, and in medicine.
It is well known in the art that plasma can be effectively used in the medical field for cutting, coagulation, and vaporization of tissues. For best results, the generated plasma has to have precise characteristics, such as velocity, temperature, energy density, etc. Preferably, plasma used for medical applications has to be pure. In other words, it should contain only particles of the ionized plasma generating gas and no other particles, such as materials separated from various parts of the plasma-generating device during operation.
Recently, attempts have been made to use plasma for tissue treatment and particularly skin treatment. Plasma may have different effects when it comes in contact with a skin surface depending on, among others, the temperature increase that it produces on the surface of the skin. For example, increasing the temperature by approximately 35°-38° C. has a wrinkle reducing effect. Increasing the temperature by approximately 70° C. removes the epidermis layer, which may be useful in plastic surgery. It has been recognized that a continuous plasma flow, suitable for cutting, coagulating, and evaporation of tissues, is not suitable for other types of tissue treatment in general, and for skin treatment in particular. Instead, to avoid undesired skin damage that would result from using continuous plasma flow, pulsed plasma is used. Two types of device that may be used for this purpose are presently known in the art.
The device disclosed in U.S. Pat. No. 6,629,974 is an example of the first type. In devices of this type, plasma is generated by passing plasma generating gas, such as nitrogen, through an alternating electric field. The alternating electric field creates a rapid motion of the free electrons in the gas. The rapidly moving electrons strike out other electrons from the gas atoms, forming what is known as an electron avalanche, which in turn creates a corona discharge. By applying the electric field in pulses, pulsed corona discharge is generated. Among the advantages of this method for generating pulsed corona discharge is (1) the absence of impurities in the flow and (2) short start times that enable generation of a truly pulsed flow. For the purposes of this disclosure, a truly pulsed flow refers to a flow that completely ceases during the off period of the pulse.
A drawback of devices and methods of the first type is that the generated corona discharge has a fixed maximum temperature of approximately 2000° C. The corona discharge formed in the device never becomes a high temperature plasma. To achieve the energy of 1-4 Joules required for modifying collagen during skin treatment by a device of this type, the rate of plasma generating gas flow has to be relatively high. For example, using argon in such a device requires a flow of approximately 20 liters/min to achieve the required energy. That flow rate is impracticable for skin treatment. When nitrogen is used for generating plasma, the required energy can be achieved with a flow rate of only about 5 liters/min, but even this rate will create discomfort for a patient. Accordingly, the applications of devices of the first type are limited by the nature of the electrical discharge process that is capable of producing a corona discharge.
Devices of the second type generate plasma by heating the flow of plasma generating gas passing through a plasma channel by an electric arc that is established between a cathode and an anode that forms the plasma channel. An example of a device of the second type is disclosed in U.S. Pat. No. 6,475,215. According to the disclosure of U.S. Pat. No. 6,475,215, as the plasma generating gas, preferably argon, traverses the plasma channel, a pulsed DC voltage is applied between the anode and the cathode. A predetermined constant bias voltage may or may not be added to the pulsed DC voltage. During a voltage pulse, the number of free electrons in the plasma generating gas increases, resulting in a decrease in the resistance of the plasma and an exponential increase of the electric current flowing through the plasma. During the off period, the number of free electrons in the plasma generating gas decreases, resulting in an increase in resistance of the plasma and an exponential decrease in the current flowing through the plasma. Although the current is relatively low during the off period, it never completely ceases. This low current, referred to as the standby current, is undesirable because a truly pulsed plasma flow is not generated. During the off period a continuous low-power plasma flow is maintained. In essence, the device does not generate pulsed plasma, but rather a continuous plasma flow with power spikes, called pulses, thus simulating pulsed plasma. Because the off-period is substantially longer than a pulse, the device outputs a significant amount of energy during the off period and, therefore, it cannot be utilized effectively for applications that require a truly pulsed plasma flow. For example, if the device is used for skin treatment, it may have to be removed from the skin surface after each pulse, so that the skin is not exposed to the low power plasma during the off period. This impairs the usability of the device.
Dropping the current flow through the plasma to zero between pulses and restarting the device for each pulse of plasma is not practicable when using the device disclosed in U.S. Pat. No. 6,475,215. Restarting the device for each pulse would result in the rapid destruction of the cathode, as a result of passing a high current through the cathode without ensuring that cathode arc attachment is well controlled.
The inability of the device disclosed in U.S. Pat. No. 6,475,215, and other devices of this type presently known in the art, to generate a truly pulsed plasma flow that can be safely used on a patient is due to the structure of the device. It takes a few milliseconds to reach a plasma flow phase after the off period. During these few milliseconds the plasma properties are not easily controlled, and therefore it cannot be used on a patient. Additionally, when devices of this type startup there is some erosion of electrodes due to sputtering. This erosion results in separated electrode materials flowing in the plasma. When a continuous plasma flow is used, the startup impurities are a relatively minor drawback, because the startup, and the impurities associated with it, occur only once per treatment. It is therefore possible to wait a few seconds after the startup for the electrode materials to exit the device before beginning actual treatment. However, waiting for impurities to exit the device when using a pulsed plasma flow is impractical, because the next pulse of plasma would have to be generated before the waiting period is over.
When the plasma flow has been previously created it takes just a few microseconds to increase or decrease the current in the plasma flow. Additionally, because there is no startup, impurities do not enter the plasma flow, and there is no stress on the cathode. However, sustaining even a low electrical current through the plasma continuously renders the device suboptimal for some applications that require a truly pulsed plasma flow, as discussed above.
Difficulties in generating a truly pulsed plasma flow by the means of heating the plasma generating gas with an electric arc are primarily due to the nature of processes occurring on the electrodes. In general, and for medical applications especially, it is critical to ensure operation free from the erosion of the anode and the cathode when the current rapidly increases. During the rapid current increase the temperature of the cathode may be low and not easily controlled during subsequent repetitions of the pulse. When generating an electric arc between the cathode and the anode, the area of attachment of the arc to the cathode strongly depends on the initial temperature of the cathode. When the cathode is cold, then the area of attachment is relatively small. After several pulses the temperature of the cathode increases, so that during the period of a rapid current increase the area of attachment expands over the entire surface area of the cathode and even the cathode holder. Under these circumstances the electric potential of the cathode begins to fluctuate and the cathode erosion begins. Furthermore, if the area of attachment of the electric arc reaches the cathode holder it begins to melt thus introducing undesirable impurities into the plasma flow.
A similar situation occurs on the surface of the anode. When the current in the arc increases rapidly, the plasma flow does not have sufficient time to reach a high temperature. As a result, the concentration of plasma close to the anode surface is low. This leads to a drop in the electric potential of the anode and its fluctuation which causes intensive erosion of the cathode. Fluctuations in the electric potentials of the cathode and anode lead to an unstable and not easily controlled energy of the pulsed plasma flow.
For the cathode to function properly it is necessary to control the exact location and the size of the area of attachment of the electric arc to the cathode surface during the periods of rapid current increase in each pulse of plasma. For the proper function of the anode it is necessary to establish the flow of the heated plasma at the surface of the anode during the rapid current increase as well as during the operational period of the pulse.
Generating truly pulsed plasma, especially for medical applications, poses several additional problems. First, as mentioned above, plasma has to be pure, free from any electrode materials or other impurities. Second, properties of the generated pulse of plasma have to be controlled. Initially, by controlling the duration, voltage and current of the pulse the energy transferred by the pulse can be controlled. For some applications, such as skin treatment, merely controlling the energy transferred in the pulse is not enough; the energy and temperature have to be distributed substantially uniformly over the treated area.
Accordingly, presently there is a need for a device that overcomes the limitations of the currently known devices by generating truly pulsed plasma with minimal amounts of impurities, and by substantially uniformly distributing energy transferred in each pulse over the treated area. Additionally, there may be applications where the device optionally needs to be capable of supplying ozone to the treated surface and removing fluids and other extraneous matters from the treated surface.